L–O

luteinizing hormone (LH) A peptide HORMONE
the anterior lobe of the PITUITARY GLANDproduces
that stimulates hormonal activity related to repro-
duction. In men, LH stimulates the development
and function of interstitial cells in the TESTESthat
synthesize and release TESTOSTERONE, the primary
male sex hormone. In menstruating women, a
mid–menstrual cycle surge of LH stimulates OVU-
LATION(the maturation and release of an ovum, or
egg). In a woman who is pregnant, the PLACENTA
also produces LH. The HYPOTHALAMUS’s release of
GONADOTROPIN-RELEASING HORMONE (GNRH) stimu-
lates the pituitary gland to secrete LH. Rising lev-
els of the sex hormones (ESTROGENS, PROGESTERONE,
andTESTOSTERONE) and INHIBINcause the hypothal-
amus to stop releasing GnHR, ending the pituitary
gland’s secretion of LH.
For further discussion of LH within the context
of the endocrine system’s structure and function,
please see the overview section “The Endocrine
System.”
See also ADRENOCORTICOTROPIN HORMONE(ACTH);
ANABOLIC STEROIDS AND STEROID PRECURSORS; ANTIDI-
URETIC HORMONE(AH); CHORIONIC GONADOTROPIN; FOL-
LICLE-STIMULATING HORMONE(FSH); GROWTH HORMONE
(GH); MENSTRUATION; OXYTOCIN; PROLACTIN; RELAXIN;
THYROID-STIMULATING HORMONE(TSH).

melatonin A peptide HORMONE that the PINEAL
GLANDsecretes, the primary function of which is to
regulate the body’s circadian cycle (pattern of
sleep and wake). The pineal gland synthesizes
melatonin from the amino acid tryptophan. The
OPTIC NERVEappears to convey NERVEmessages of
outside light and dark from the RETINAto a section
of the HYPOTHALAMUScalled the suprachiasmatic
nucleus (SCN). The SCN sends nerve signals to the

pineal gland, which suspends melatonin synthesis.

Darkness causes the nerve messages from the

optic nerve to stop, which in turn ends the signals

from the SCN. When receiving signals from the

SCN, the pineal gland stops melatonin production.

When the signals from the SCN end, the pineal

gland resumes melatonin production. Researchers

believe melatonin causes sleepiness by slowing cell

METABOLISM. Other endocrine processes, such as

the adrenocorticosteroid hormonal cascade that

regulates CORTISOLlevels, also slow in conjunction

with the circadian cycle, though researchers are

uncertain about how these processes may be

interrelated.

For further discussion of melatonin within the

context of the endocrine system’s structure and

function please see the overview section “The

Endocrine System.”

See also MELATONIN SUPPLEMENT; STRESS RESPONSE

HORMONAL CASCADE.

multiple endocrine neoplasia (MEN) An inher-

ited genetic disorder in which numerous tumors

form in various endocrine glands. Several GENE

mutations are responsible for the errant growth of

glandular tissue, which may take the form of

tumors or hypertrophy (overgrowth). Oversecre-

tion of the affected gland’s hormones then occurs.

The three forms of MEN are

• MEN-1, in which adenomas develop in the
  PITUITARY GLAND, PARATHYROID GLANDS, and ISLETS
  OFLANGERHANSin the PANCREAS


• MEN-2a, characterized by HYPERPARATHYROIDISM,
  PHEOCHROMOCYTOMA, and medullary THYROID
  CANCER

151